ANATOMY OF VERTEBRATES. 



fig. 1, df, and into the aponeurosis called ' centrum tendineum' 

 or 'cordiform tendon,' ib., t. This centre is widely notched 

 toward the spine, and divided anteriorly into three tracts, of which 

 the right is usually the largest. Between the right and middle tracts 

 is the orifice, c, for the inferior vena cava (< postcaval ' of Mam- 

 mals). Behind the tendon, and to the left of the median line, is 

 the orifice, e, for the oesophagus and pneumogastric nerves : the 



aorta, «, passes from the 

 chest to the abdomen be- 

 tween the f crura ' of the 

 lesser muscle. The right 

 1 crus ' in Man arises from 

 the three or four upper lum- 

 bar vertebrae ; the left crus 

 does not descend so low : 

 both muscular bundles ex- 

 pand as they rise, decus- 

 sate at the oesophageal open- 

 ing, and are inserted into 

 the posterior concavity of 

 the central tendon and in- 

 ternal ligamentum arcua- 

 tum, fig. 1,/. 



The diaphragm is most muscular, longest, and most oblique in 

 Cetacea, in which the central tendon is almost obsolete : by rising 

 so far back, it permits the proportional extension of the lungs, 

 which in the Dugong and Manatee act as air-bladders. In the 

 perissodactyle Ungulates, in which the moveable ribs are numerous 

 and continued to near the pelvis, the diaphragm is also extensive, 

 and much arched toward the thorax. 



§ 193. Muscles of Monotremata. — To give an account of the 

 muscular, as fully as that of the osseous, system of the Mammalia, 

 would not be attended with the same advantages, even if a detailed 

 myology comported with the scope and extent of the present work. 

 This part of Mammalian anatomy will therefore be limited to the 

 notice of a few select examples. Fig. 3, from Meckel, 1 shows the 

 more remarkable muscles of the Ornithorhynchus. The animal is 

 dissected from the ventral surface ; the great ( panniculus carnosus,' 

 l, is reflected from the right side, and the deeper-seated muscles 

 are shown on the left. The panniculus carnosus, which is remark- 

 able for its thickness, encompasses nearly the whole body, adhering 

 most firmly to the external skin, but separated from the subjacent 

 muscles, especially where it covers the thorax, abdomen, the arm, 



Human diaphragm. Thoracic surface from behind. 



